Generally, a paper sheet processing apparatus sandwiches and conveys a paper sheet that is a processing object by conveyor belts. In addition, the paper sheet processing apparatus distinguishes and sorts the paper sheet. In the paper sheet processing apparatus, a sensor that detects a position of the paper sheet under conveyance is installed in a conveying way. Usually, a transmission type photoelectric sensor is used as the sensor. The transmission type photoelectric sensor has a light-emitting device and a light-receiving device. The light-emitting device and the light-receiving device are installed so that they may oppose via the conveying way. The transmission type photoelectric sensor detects existence of the paper sheet by detecting that the paper sheet interrupted a light emitted from the light-emitting device.
In recent years, as a processing object, transparent-envelope sealed letters which have a transparent envelope and the contents inserted in the transparent envelope increase in number. The transmission type photoelectric sensor cannot detect the transparent envelope because the light from the transmission type photoelectric sensor penetrates the transparent envelope.
In a case that a processing object under conveyance is sorted, a time when a tip end of the processing object is detected is used as an operation timing of a sorting gate. When the tip end of the processing object is the transparent envelope, operation of the sorting gate cannot start until an opaque portion is conveyed. Therefore, the operation timing of the sorting gate is overdue, and as a result, a trouble that the processing object can not be sorted occurs.